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I know all turtles are going to be different, but approximately how long can I expect a 75 gallon tank to be sufficient? (Hatchling -> 7.5")

That depends on what/how much you're feeding. My 2 hatchling RES are 1 year old and already about 5 inches each, due to overfeeding in the first few months. I researched about all the other care requirements and kind of left food til last. I've read on several sites that growth of 1 inch per year is ideal. If you can manage that, you're looking at 5-7 years (leaving room for error here).

Forgot to mention if you end up having a male (just one turt right?) then he'll be good in there for life, although bigger is always better. 10 gallons per inch is the minimum only.

Yeah, planning on just having one at the moment. I would ideally like to only purchase 1 tank, but if I can have long enough before it outgrows the tank, an upgrade down the road is possible. I've found a 75g for $150. The 90 is double that and the 120 is even worse.

If you're careful with feeding it should run you at least a few years. That's plenty of time to save up some extra cash for an upgrade down the road. Get a long tank instead of the regular tall fish tank. More surface area is better. Turts swim round and round and back and forth more than they swim up and down.

Thanks for the info.